So, if you don't know what I'm talking about, I'm going to give you a link to a video of people bouncing a basketball around (you'll need Java installed). You should start the video and count how many times the players in white pass the ball.
Anyway, I was talking about this video with Max Maven. He mentioned that some while ago someone showed the clip to a large gathering a magicians, and that there were four magicians in the large group who noticed the trick:
Max theorized that he and Penn noticed it because they (and their acts) tend to be multitaskers; they were able to deal with the task at hand (counting the passes) while still being aware of the rest of the surroundings. Max also theorized that Lance and David noticed it because they were just really tired from having done recent performances and instead of trying to count the number of passes, they were just relaxing and letting their mind wander.
I have no reason to doubt that theory, not knowing any of the other three magicians personally.
I myself did notice it, but only because the context in which I was presented the video was a bit suspect, having seen many Prank Flashes recently. I do wonder if someone extremely used to seeing Prank Flashes would instantly notice the trick even without suggestive context, merely from being used to something unexpected happening all the time.
Of course, even if they do claim to have noticed it on the first pass, who would believe them?